In recent years, in view of savings in resources and conservation of energy, there have been increasing demands for nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, lithium ion secondary batteries, etc. which can be used repeatedly. Among them, lithium ion secondary batteries are characterized by lightness in weight, high electromotive force, and high energy density. Thus, there are growing demands for the lithium ion secondary batteries as power sources for driving various kinds of mobile electronic devices and portable communication devices such as mobile phones, digital cameras, video cameras, and laptop personal computers.
On the other hand, to reduce used amount of fossil fuel, and to reduce the amount of emission of CO2, expectations for battery packs are growing to serve as power sources for driving motors such as vehicles. Such a battery pack includes a plurality of battery modules in which a plurality of batteries is connected in series and/or in parallel in order to obtain a preferable voltage and capacity.
Here, when a problem such as generation of excessive heat occurs in a battery included in the battery modules, thermal runaway, or the like may cause a serious problem such as rupture of the battery.
For this reason, an example of a battery pack used as a power source of a personal computer, or the like is disclosed, where the battery pack includes batteries connected in series by a thermal fuse, or the like (see, for example, PATENT DOCUMENT 1). Moreover, a mechanism was provided in which when an external short circuit occurs between external terminals of the battery pack, an overcurrent is detected by a protection IC provided on a safety protection circuit board, and a current is cut off by a FET device. However, since the thermal fuse is connected to leads of the batteries by soldering, or the like, control is required so that the thermal fuse is not blown at a temperature at the time of the connection, which causes problems in workability and productivity.
For this reason, a battery pack including a current cutoff mechanism is disclosed (see, for example, PATENT DOCUMENT 2). In the current cutoff mechanism, a first connection plate is connected to a second connection plate by a contact, around which a foaming layer having insulating properties is provided, and the thickness of the foaming layer is increased by excessive heat generated in the battery so that the contact is opened.
Moreover, an example of a battery pack in which PTC elements and fuses are connected in series is disclosed, wherein the fuses are formed in a stripe pattern of a thin film on an insulating substrate (see, for example, PATENT DOCUMENT 3).